This invention relates to a method for manufacturing dense nanocrystalline ceramic articles, and in particular, to a hot pressing method that simultaneously uses high compaction pressures in the range of about 1.5-8 GPa and low sintering temperatures in the range of about 0.2-0.6 Tm where Tm is the absolute melting temperature to consolidate amorphous or nanocrystalline powder compacts into fully dense nanocrystalline ceramic articles.
Methods have been developed for the production of nanostructured oxide and non-oxide ceramic powders. Nanopowders have been formed by rapid condensation of precursor species from the vapor state. However, processing these powders into bulk nanocrystalline forms has proven to be difficult, due to complications arising from the presence of high levels of chemisorbed species on the nanoparticle surfaces, as well as the occurrence of severe interparticle friction effects during powder compaction. Even if these difficulties are overcome, high temperature sintering of green state compacts often results in rapid grain growth, thus losing the opportunity to realize a bulk nanocrystalline structure. This effect can be mitigated to some extent by adding grain growth inhibitors, but usually at the expense of degrading mechanical and/or physical properties.
Various approaches to the consolidation of ceramic powders have been investigated, with varying degrees of success. Microwave-assisted sintering of nanopowder compacts of xcex3-Al2O3 is examined in J. Freim et al., xe2x80x9cMicrowave Sintering of Nanocrystalline xcex3-Al2O3xe2x80x9d, Nanostructure Mater., 4, 371-385 (1994). It was observed that the occurrence of a phase transformation (xcex3-phase to xcex1-phase) during sintering without an applied pressure resulted in rapid grain growth, which hindered further densification. Hot pressing of xcex3-Al2O3 and rapid sintering at 1125-1175xc2x0 C. is examined in S. J. Wu et al., xe2x80x9cSintering of Nanophase xcex3-Al2O3 Powderxe2x80x9d, J. Am. Ceram. Soc., 79, 2207-11 (1996) which described much reduced sintering rate at higher temperatures. At 1350xc2x0 C., a relative density of 0.82 was realized. In another approach described in R. S. Mishra et al., xe2x80x9cHigh-Pressure Sintering of Nanocrystalline xcex3-Al2O3xe2x80x9d, J. Am. Ceram. Soc., 79, 2989-92 (1996), xcex3-Al2O3 was sintered at 650-1100xc2x0 C. under high pressure (xcx9c1 GPa). The xcex3 to xcex1 transformation temperature was reduced from 1200xc2x0 C. at ambient pressure to about 750xc2x0 C. at 1 GPa. Fully dense compacts with a grain size of about 142 nm were obtained at 1000-1100xc2x0 C. and 1 GPa in 10 minutes. The transformation and densification of nanocrystalline xcex8-alumina during sintering forging is described in C. S. Nordahl et al., xe2x80x9cTransformation and Densification of Nanocrystalline xcex8-Alumina during Sinter Forgingxe2x80x9d, J. Am. Ceram. Soc., 79, 3149-54 (1996). It was demonstrated that by using seeded nanocrystalline xcex8-alumina as the starting material, dense xcex1-Al2O3 with a grain size of 230 nm could be obtained at 235 MPa/1060xc2x0 C. for 30 minutes. The compaction and heat treatment behavior of nanocrystalline (xcx9c20 nm) xcex3-Al2O3 at high pressure is described in M. R. Gallas et al. xe2x80x9cFabrication of Transparent xcex3-Al2O3 form Nanosize Particlesxe2x80x9d, J. Am. Ceram. Soc., 77, 2107-12 (1994). Pressures up to 3 GPa and liquid nitrogen as a lubricant were utilized to form transparent green-state compacts, followed by pressure-less sintering at 800xc2x0 C. for a few hours. Transmission Electron Microscope (TEM) examination of the sintered sample revealed a random dense-packed particle structure and interconnected porosity. Interstitial void dimensions, however, were always less than the average particle diameter.
Approaches described in S. C. Liao et al., xe2x80x9cHigh Pressure and Low Temperature Sintering of Bulk Nanocrystalline TiO2xe2x80x9d, Materials Science and Engineering A, 204, pp. 152-159 (1995); xe2x80x9cTheory of High Pressure/Low Temperature Sintering of Bulk Nanocrystalline TiO2xe2x80x9d, Acta Materialia, 45 [10], pp. 4027-4040 (1997); xe2x80x9cRetention of Nanoscale Grain Size in Bulk Sintered Materials via a Pressure-Induced Phase Transformationxe2x80x9d, Nanostructured Materials, 8[6], pp. 645-656 (1997); and xe2x80x9cThe Effect of High Pressure on Phase Transformation of Nanocrystalline TiO2 during Hot-Pressingxe2x80x9d, Nanostructured Materials, 5[3], pp. 319-325 (1995) found that bulk TiO2with nanoscale grain structure could be produced by high pressure/low temperature sintering of a metastable TiO2 polymorph (anatase). The high pressure was described as not exceeding 1.5 GPa in these works and the low temperature was described in the range of 400-445xc2x0 C. The combined high pressure and low temperature sintering was found to reduce the diffusion rate while increasing the nucleation rate of the stable phase (rutile). The net result was the production of nanocrystalline (xcx9c36 nm grain size) TiO2 (rutile) with high density ( greater than 98%), high hardness (800 VHN, and about 6 times improvement in wear resistance. See Y. Iwai et al, xe2x80x9cTribological Properties of Nanocrystalline TiO2xe2x80x9d, Proceedings of JAST Tribology Conference, Osaka, November, 1997, Japanese Society of Tribologists, Osaka, Japan, 1997, 1997, pp. 209-212.
It is desirable to provide nanocrystalline ceramics with improved hardness and wear resistance, combined with good fracture toughness, compared with their microcrystalline counterparts.
The present invention relates to a method for fabricating nanocrystalline ceramic articles in which a loosely agglomerated ceramic nanopowder is synthesized. For example, the nanopowder can be synthesized by chemical vapor condensation (CVC) using a hot-wall reactor for the production of non-oxide ceramics and a combustion-flame reactor for the production of oxide ceramic powders. The nanopowder is formed into a compact and consolidated at pressures of at least 1.5 GPa and temperatures of no greater than 0.6 times the absolute melting temperature of the ceramic nanopowder (Tm). For example, the nanopowder can be compacted at pressures in the range of 1.5-8 GPa and temperatures in the range of 0.2-0.6 Tm.
It has been found that high compaction pressure causes deformation, such that the green density increases with pressure up to a maximum at about 8 GPa. Low sintering temperature mitigates grain growth during consolidation. The simultaneous application of high pressure and low temperature to a nanocrystalline powder compact under near isostatic conditions produces a sintered nanophase ceramic article with high density and a grain size comparable with the original powder particle size. The article can be produced with superior properties and performance for specific applications. For example, higher strength and toughness is advantageous for components in combustion and gas turbine engines, higher hardness and wear resistance is advantageous for protective coatings, and enhanced optical transparency is advantageous for infra-red windows, aircraft canopies, and high intensity lamps.
In one aspect of the invention nanophase powder with a metastable structure can be used as a starting material. Under high pressure, the metastable phase transforms to a more stable phase, which effect promotes the consolidation process. This transformation-assisted consolidation has been successfully applied to produce sintered oxide and non-oxide bulk nanocrystalline ceramics having a grain size of less than 100 nm, starting with even finer-scale ceramic nanopowders in the range of 5 to 50 nms. Also, under appropriate conditions for example sintering pressures in the range of 3 GPa to 5.5 GPa, the grain size of the nanocrystalline sintered product can be smaller than the original nanopowder particle size. These effects have not previously been reported, since prior work on sintering of ceramic powder compacts has been limited to pressures less than 1.5 GPa.
Although the method of the present invention is especially useful for processing nanophase Al2O3, TiO2, Y2O3 and Si3N4, other types of oxide and non-oxide ceramics, such as YSZ, BaTiO3, YBCO, AlN, SiC and their composites, can also be processed using the method of the present invention. It is typically advantageous to utilize high quality ceramic nanopowders produced by vapor condensation methods as starting materials, because of their high purity, low degree of agglomeration, and extraordinary susceptibility to sintering. The latter is simply a reflection of the high driving force for sintering due to the high surface area of the nanoparticle compacts. It has been found that the exceptionally fine grain sizes of the nanocrystalline ceramics formed by the method of the present invention give improvements in properties and performance that are unattainable with conventional microcrystalline ceramics. Accordingly, a preferred embodiment of the present invention is a dense nanocrystalline ceramic material that has a completely uniform grain size of less than 100 nm in the fully sintered state.
In one embodiment, a near-net shape forming of the ceramic article can be accomplished by using a soft mold technique, wherein the shaped compact is subjected to a uniform isostatic pressure by surrounding it with a pressure transmitting medium, such as boron nitride powder. By limiting the exposure time at the sintering temperature, using a grain growth inhibitor, or exploiting a phase transformation during sintering, the initial nanophase structure can be substantially retained, or even reduced, during the consolidation step in the process, thereby ensuring superior properties in the finished article.